Hi there mommy, this article actually counters what your mom says. M position, according to the article, is the ideal position as it centers the femur head right in the middle of the hip socket and thus aids the development of baby's hips.
http://www.babywearingadvice.co.uk/anatomy.htm
As for the cradle sling, it is up for debate. Here's an article that talks about the pros and cons of the cradle sling usage on young babies.
http://www.sleepingbaby.net/wearing/cradle.php